Dark Night Focus

[bjk896]

I searched without success ...


Night focussing for star trails or any atrophotography can be a hard task. I recently bought a 4 point cross lens to use. It screws on like a normal filter and I remove it when Im happy with focus on some significant star and start real shooting.
The idea is that the 4 star points can give a good indication of focus with me refining the manual focus ring it until the crosses are crisp. Some say to use screen magnification to help it all along.
Has anyone had success with this technique and maybe a process to streamline the time taken and also mark or identify the focus ring position for later shots.

[Manfred M]

That is not a technique I have ever heard of. I know a number of good star shooters and they use several different techniques and I can't recommend one over the other as I have only used two of them (number 2 and 3). I suspect all are going to work better than the star burst filter.

1. Pre-focus your lens at an object that is very far away during daylight hours and tape down your lenses focus ring at that position with gaffers tape (which does not leave a sticky residue) to lock down that position. If you have a zoom lens, pre-select your focal length and tape down the zoom ring as well.

2. If your lens has distance markings, focus on a distant object during daylight hours and record where the lens focuses at infinity. Either remember where that is or take a picture of your lens showing the alignment between the infinity marking and the focus indicator line. When you are ready to shoot, get a small light to let you dial that in manually.

3. Get a manual focus lens. These have a hard stop at the infinity setting and just rotate the focus ring until it hits the stop.

[xpatUSA]

... and also mark or identify the focus ring position for later shots.

I would not trust existing or added focus marks on any lens, including older manual focus lenses. The position travel at or around infinity is too sensitive. Moreover, most if not all lenses focus are affected by temperature. I can't imagine presetting during the day in the Mojave desert and then shooting at night being reliably accurate.

I do not do astrophotography but, if I did, I would be inclined to use magnified live view and to buy a camera with the best possible LCD resolution or one which can tether to a high-resolution external monitor.

As to star filters, it would be interesting to know more about how that works - specifically as a focusing aid.

https://hoyafilterusa.com/how-star-filters-work/
.

[Manfred M]

I do not do astrophotography but, if I did, I would be inclined to use magnified live view and to buy a camera with the best possible LCD resolution or one which can tether to a high-resolution external monitor.

Too much on screen noise in using this approach Ted. One has to boost the magnification on the LiveView fairly high. It's hard enough to use this technique on relatively dimly lit buildings at night (it is a technique I use). Seeing the stars? One can't make them out.

The mark technique works well. Again, something I can confirm from personal use for other low light photography of distant objects.

[xpatUSA]

Too much on screen noise in using this approach Ted. One has to boost the magnification on the LiveView fairly high. It's hard enough to use this technique on relatively dimly lit buildings at night (it is a technique I use). Seeing the stars? One can't make them out. ...

Thanks for responding, Manfred.

[Stagecoach]

I do not do astrophotography but, if I did, I would be inclined to use magnified live view

I have found this method successful Ted when you select a very bright star as your target. It is then easily distinguished from the noise.

Hard stops, taping and pre-focusing can work but each have their risks.

[rpcrowe]

I also don't do astrophotography but, wonder if Focus Assist (magnified focus) along with Focus Peeking in manual focus might not be a way to get the best focus... What do you think?

[Barnwell]

I did some astrophotography and had the same focus problems. I saw this article and it seems a Bahtinov Mask may help. In the article they made their own mask but they seem to cost about $15. I have not tried this. https://petapixel.com/2017/06/23/imp...bahtinov-mask/
Read the comments below the post too.
Another device (filter) https://www.lonelyspeck.com/sharpstar/

[bjk896]

I thank you for your comments but feel the need to restate the question which, it appears, none of you undestand.
The question was does anyone have a process for the 4 point cross lens that works for dark night focus. Whilst the diversions were interesting, my question still stands - please. If you want to offer and discuss other alternatives I'd appreciate you starting another thread which isnt of interest to me. I have been thru those other methods.

[xpatUSA]

I thank you for your comments but feel the need to restate the question which, it appears, none of you undestand.

Brian, I did understand the question so kindly exclude me from "none of you". In post #3, I said:

"As to star filters, it would be interesting to know more about how that works - specifically as a focusing aid."

By which I meant why it works. Knowing that, we might be able to offer more specific advice.

[Manfred M]

I thank you for your comments but feel the need to restate the question which, it appears, none of you undestand.
The question was does anyone have a process for the 4 point cross lens that works for dark night focus. Whilst the diversions were interesting, my question still stands - please. If you want to offer and discuss other alternatives I'd appreciate you starting another thread which isnt of interest to me. I have been thru those other methods.

Brian - these threads do have a habit of going off track a bit, it's just the nature of how they evolve.

To review what has been written no one reading your question has heard of this technique so can't offer any meaningful advice.

I asked a couple of astrophotographers at one of the photo clubs I belong to about it. They hadn't heard of it either and the closest I got to an answer from one of them is "I can see how that might work".

There are a lot of very specialized techniques out there, but CiC is very much a generalist photographic site. I suspect you might have to look elsewhere for this highly specialized answer.

[proseak]

You might try looking/posting here?

https://www.dpreview.com/forums/1059

[bjk896]

Sadly, I cannot find the website where I first got the inspiration for the cross lens - and some technical detail that went wit it. However a brief description within the thread on https://photo.stackexchange.com/ques...trophotography might help the curious.
I suspect it would be more effective on a telescope than DSLR but the crosses create a star effect that can be "adjusted" into clarity which is focus for that particular light source

[Manfred M]

Sadly, I cannot find the website where I first got the inspiration for the cross lens - and some technical detail that went wit it. However a brief description within the thread on https://photo.stackexchange.com/ques...trophotography might help the curious.
I suspect it would be more effective on a telescope than DSLR but the crosses create a star effect that can be "adjusted" into clarity which is focus for that particular light source

Thanks for posting the link. An interesting read.

Question: What camera and lens are you looking at using for this astro work?

Using a long focal length telescope equipped with a sidereal time tracking motor is going to be a lot different than using a relatively fast, shorter focal length lens on a stationary tripod. Motion blur from the earth's rotation is going to impact sharpness of your images, unless you are trying for star streaks.

[bjk896]

Manfred, the camera(s) will be a Canon "OLD" 300D and/or a 760D. I have the (ebay) filter and waiting for a suitable night.

[Manfred M]

Manfred, the camera(s) will be a Canon "OLD" 300D and/or a 760D. I have the (ebay) filter and waiting for a suitable night.

Thanks - I had also asked about lens(es), which is the important part of this discussion. The focal length(s) you are planning to use are the important consideration.

[bjk896]

Thanks - I had also asked about lens(es), which is the important part of this discussion. The focal length(s) you are planning to use are the important consideration.

Sorry for the delay in reply
The lens is the standard 18-55mm canon and I'll be using it in an untracked mode with 100 or so shots at high ISO for , say, 10 seconds depending on inclination and then using one of the stacking programs to finalise it into a single photo. So it'll probably be at the shorter end but won't matter. Using 100 or so shots and stacking them eliminates any movement issue. There'll be some dark frames in there as well. Will be an interesting exercise.

[Manfred M]

Thanks - so you are looking at fairly standard wide-angle star-streak shot.

My suspicion is that you are overthinking this as the depth of field is going to quite forgiving in this arrangement, especially if you stick to the wider side of those focal lengths, which is typically done in this type of photography. Take a look at the hyperfocal and depth of field charts for the apertures and focal lengths you will be using and prefocus on a subject that will be viewed as "at infinity". You are not looking at trying to get "point" stars, but will end up with streaks, which will not be sharp anyways.

[steveg4iwr]

I searched without success ...


Night focussing for star trails or any atrophotography can be a hard task. I recently bought a 4 point cross lens to use. It screws on like a normal filter and I remove it when Im happy with focus on some significant star and start real shooting.
The idea is that the 4 star points can give a good indication of focus with me refining the manual focus ring it until the crosses are crisp. Some say to use screen magnification to help it all along.
Has anyone had success with this technique and maybe a process to streamline the time taken and also mark or identify the focus ring position for later shots.

That is very similar to the Bahtinov mask I use on the front of my telescope to ensure focus is accurate. I also have a small Bahtinov mask for my camera lens and focus manually. Never use the infinity stops or markings as they are rarely accurate, manual focus is better. I would say foolproof but I have mis focussed more than once hence using the Bahtinov mask.

[William W]

. . . The lens is the standard 18-55mm [F/3.5~5.6] canon and I'll be using it in an untracked mode with 100 or so shots at high ISO for , say, 10 seconds depending on inclination and then using one of the stacking programs to finalise it into a single photo. So it'll probably be at the shorter end but won't matter. Using 100 or so shots and stacking them eliminates any movement issue. There'll be some dark frames in there as well. Will be an interesting exercise.

I do not understand.

How does using a camera NOT on a tracking device making 100 frames and then stacking them eliminate movement?

A Night Sky scene in any location in S.A. will show a minor shift in alignments at about each 120 seconds – so how do you stack 100 frames to eliminate movement issues? I suppose using a very wide lens assists, but "100 or so shots" at 10 seconds exposure each is quite period of time elapse which means that's a lot of alignment movement, as I understand it: maybe I got it wrong, hence my not understanding your intended method or what you mean by eliminating “movement issues”.

Maybe you could define what you mean by “movement issues”, please.

WW